High-purity gallium and gallium compounds find increasing use, mainly in the electronics industry. Doped gallium arsenide, gallium phosphide and gallium arsenide phosphite have proved especially suitable as semi-conductor materials. In addition, gallium arsenide can be used in the construction of lasers and solar cells. Another possible use is in the field of superconductors based on gallium, which is currently in the development stage.
In the minerals available, gallium is present only in very low concentrations of from 0.001% (bauxite) to a maximum of 1.8% (germanite). It is generally in association with zinc, aluminium and/or copper. The low concentrations of the ore make particular demands on any process for recovering gallium as regards selectivity of the material separation and the possibility of concentration.
Gallium is obtained when working up bauxite and also zinc- and germanium-containing ores (cf. "Ullmanns Encyklopadie der Technischen Chemie", volume 11 (1986), p. 753). The separation and concentration of gallium is generally carried out in the so-called Pechiney, Alcoa or Reynolds process (cf. US-PS 3 890 427). All these processes are based on the separation of the aluminium by precipitation. From the solutions that remain, the gallium and residues of aluminium are precipitated in the form of hydroxides by means of carbon dioxide. The aluminium- and gallium-containing hydroxide is dissolved in sodium hydroxide solution and subjected to electrolysis. Further, a solution having a low concentration of gallium is obtained as a circulating solution of the Bayer process.
The hydrometallurgical separation of gallium has hitherto been carried out by liquid-liquid extraction from hydrochloric acid solutions (cf. M.L. Good, F.F. Holland, "J. Inorg. Nucl. Chem.", 26 (1964), page, 321, and T. Sato, T. Nakamura, S. Ishikawa, "Solv. Extr. I. Exch."2 (1984), page 2019). The use of tributyl phosphate, triaryl-phosphine, trioctyl phosphine oxide, crown ethers or dihexyl sulphide for the extraction of gallium is also already known (cf. V.P. Judin, R.G. Bautista, "Metall. Trans. B. ", 17B (2), 1986, page 259; A.M. Reznik, L.A. Zekel, "Zh Neorg. Khim." 24 (4), 1979, page 1025; Y. Amashji, T. Matsushita, M. Wada, T. Shono, "Chem. Lett." 1 (1988), page 43; Y. Hasegawa, T. Shimada, M. Niitsu, "J. Inorg. Nucl. Chem." 42 (10), 1980, page 1487; H. Koshima, H. Onishi, "Analyst (London)" 111 (11) (1986), 1261; and Y. Baba, H. Nakamura, K. Inoue, "J. Chem. Eng. Jpn." 19 (6), 1986, page 497).
The yields of high-purity gallium that can be obtained by these processes are, however, inadequate, and in particular the complete removal of aluminium, zinc or copper is insufficient if these are in a high excess compared with gallium.
The problem underlying the invention was therefore to find a process rendering possible the separation and concentration, in an even more effective manner, of gallium from aqueous solutions containing gallium together with aluminium, zinc and/or copper.
It has now been found that this problem can be solved in accordance with the invention by separating and concentrating the gallium in a multiple emulsion system using secondary amines as selective transporting reactants. That is to say, when the hydroxides (for example from working up bauxite according to Pechiney) or ores containing aluminium and gallium in addition to other heavy metals are leached with hydrochloric acid solution, it is possible to separate and concentrate gallium by reaction extraction using liquid membrane emulsions. By this means concentrated gallium solutions of a high degree of purity are obtained.